i was thinking along the lines of the old windlass cross bows
using a coil to store torsional power wich in turn spins the spindle/wheel

the rail part is something i thought up later on, after i thought of post apoc. viability
since having a way to use almost anything as ammo would be an edge in a senario where resources might be gone/limited
along with the stealth advantages of a non firearm

if i forgo the bullcrap if the rail and jus have an internalised crossbow (no risers)
what way would be cheaper, easier or more durable/reliable

my main choice of ammo would be marbles, bearings, and bolts
and my goal is to have a weapon capable of killing medium game
(this is not intended to be a prop or used outside of target shooting or perhaps hunting)

also messing with the idea of a tether/winch system for using bolt harpoons